Hello
The previous owner of my home replaced the old dinosaur of a furnace with a Ducane High Efficency furnace. It is vented to the outside via PVC pipe in the foundation as it should be. My question is about the cold air return. The installers left the original monstrous cold air return in place. It is a large opening roughly 2ft wide and 3 1/2ft long. I also noticed that to the right of this monstrous return is a smaller grate that ducts into the same large return. The grate is missing off of the larger return, it is utilizing space in my living room that I would like to use. and I would like to at least scale it down if I could. Is this possible with the other small return next to it? I was planning on leaving the box the same size just covering it but not sure if the small return would be enough.

Thanks

Most likely you can downsize the return duct. However before you go about this we need to know a few things.
How many BTU's is the furnace. And if you have central AC installed what size it that as well?

Once we know we can determine what is the minimum size duct opening and grill opening you can use.

Hello I am sorry it took so long to reply but there are some more issues.

As far as I could see on the unit, I cant find any BTU listing just the model number and serial. The model is Ducane CMPB075U3. It does have a central air unit attached but cant find a size on it either; although I will say that it is a smaller unit also made by Ducane.

In looking at the furnace I did notice that there is one vent pipe that is running through the foundation wall that is for the flue products, but there is another pipe that is coming off of the top of the furnace that is open. After looking at a diagram I find that this is the combustion air inlet. It dosent seem to be pulling much if any air but am curious to see if that is normal or not. The furnace seems to run normally. The reson i bring these things up is that I dont want to make any modifications to the Cold Air Return and find that something wasnt installed correctly and I cause problems.

Thanks again

The furnace is 75,000 BTUs, fairly small, so I assume the house is also.

With that size furnace the max AC system would be around three tons. Unless you had a oversized fan unit in the system.

Three tons of AC require 1200 CFM of air to properly operate.

You would need a 24 x 18 return grill to allow for this air to get back to the furnace.
But that is just a gill alone based on how much free air it can pull through it. Add a filter to this mix and this grill should be @ 24x24 in size.
This equates to 576 square inches of grill surface. So you can use any amount of grills as long as the combined face surface adds up to 576 inches. But this is the least amount of size to use. Any additional free air allowed is better.
Duct size also needs to be considered.
20" x 8" is smallest size duct to run back to furnace. Larger is better.

The reason for larger being better both on the filter grill size and duct is air velocity. The smaller the duct the faster the air needs to run through it. Thus the more noise. In the filter area if the air runs to fast through it, it fails on trapping the fine dust. So if the air can move slower through the duct and filter you will have a quite system with good filtering of dust.


As far as the return combustion air pipe.
If your home is fairly old and not to air tight, not a big deal. But having a high efficency home with little outside air influence this pipe should be run outside for the best of energy savings.
You will not feel to much air going into the pipe, but I assure you there is just as much going in it as you feel coming out the exhaust end.

Ok thanks for all the input. I will take a look at things, get some acurate measurements and take it from there.

Once again Thank you